A method which has been used to increase the capacity of cellular communication systems is the concept of hierarchical cells wherein a macrocell layer is underlayed by a layer of typically smaller cells having coverage areas within the coverage area of the macrocell. In this way, smaller cells, known as microcells or picocells (or even femtocells), are located within larger macrocells. The microcells and picocells have much smaller coverage thereby allowing a much closer reuse of resources. Frequently, the macrocells are used to provide coverage over a large area, and microcells and picocells are used to provide additional capacity in e.g. densely populated areas and hotspots. Furthermore, picocells can also be used to provide coverage in specific locations such as within a residential home or office.
The current trend is towards introducing a large number of small picocells to 3G systems. For example, it is envisaged that residential access points may be deployed having only a target coverage area of a single residential dwelling or house. As another example, it has been proposed to cover e.g. office buildings in a number of small picocells with a range of a few tens of meters. Such systems may specifically be marketed as enterprise systems allowing a given enterprise, such as a company or building administrator, to provide communication services with a high capacity, high flexibility and low cost.
However, underlaying a macrolayer of a 3rd Generation cellular network with a picocell (or microcell) layer raises several issues. For example, the introduction of a large number of underlay cells creates a number of issues related to the identification of individual underlay cells when e.g. handing over to an underlay call. In particular, 3rd Generation cellular communication systems are developed based on each cell having a relatively low number of neighbours and extending the current approach to scenarios wherein the mobile phone may need to consider large numbers of potential neighbour cells is not practical.
One problem of extending current approaches to scenarios where there are many underlaying picocells is how to uniquely and efficiently identify a picocell (or microcell). Specifically, it is not practically feasible to list every underlay cell as a potential neighbour of the macrocell as this would require very large neighbour lists. These large neighbour lists would e.g. result in the neighbour list exceeding the maximum allowable number of neighbours in the list, slow mobile station measurement performance as a large number of measurements would need to be made etc. It would furthermore require significant operations and management resource in order to configure each macrocell with a large number of neighbours. However, sharing identification codes (e.g. scrambling codes) for the pilot signals of the picocells results in a target ambiguity and prevents the mobile station uniquely identifying a potential handover target.
Also, the introduction of a large number of access points/base stations supporting underlay cells introduces a number of issues relating to routing and addressing within such a network. In particular, the current hierarchical addressing used in cellular communication systems has a very limited address space and does not allow an unlimited number of nodes to be introduced. In addition, it is important that routing and management operations retain a very high degree of security and mobile authentication which becomes increasingly difficult when needing to accommodate a large number of distributed nodes.
Specifically, in a current macrocell cellular system, the address system is defined with a balance between scope and speed of resolution for the expected architecture hierarchy. For example, in UMTS, only 4096 unique addresses are available for Radio Network Controllers (RNCs). Specifically, the Iu interface connecting the RNCs to the Core Network (CN) uses the Signalling System 7 (SS7) protocol which has 4096 addresses known as Signalling Point Codes (SPCs) available for RNCs thereby limiting the total number of unique RNC addresses to 4096. Furthermore, a UMTS system typically has a limit on base station addresses available at each RNC.
The approach of managing the address resolution in a hierarchical fashion whereby a cell or mobile station is addressed by a set of fixed scope network address levels works well when the expected hierarchy relationships are met, e.g. around 100 Cells per RNC, and no more than a few thousand RNCs per operator.
However, the approach is unsuitable for systems where the number of nodes at a given level exceeds the address scope for that level. For example, the introduction of large numbers of base stations/access points supporting very small cells means that the number of cells may exceed the address scope, and it is accordingly not possible to resolve certain addresses within the defined scope.
Furthermore, in some cases it has been proposed that individual residential or enterprise access points include at least some RNC functionality such that the individual residential access point is coupled to the network as an RNC entity with an individual RNC identity. However, as many tens of thousand residential or enterprise access points may exist in a given network, this substantially exceeds the address space available for RNCs.
Hence, an improved radio access network would be advantageous and in particular a network allowing increased flexibility, improved addressing, increased address scope, secure operation, improved handovers, improved support for large numbers of underlay cells, improved suitability for large numbers of potential handover target cells, improved suitability for underlay/overlay handovers, reduced neighbour lists, increased practicality, reduced measurement requirements and/or improved performance could be advantageous.